How much does replacing windows really cost?

When is the renewal of the windows economically viable?

But when are the costs of renewal of the windows worthwhile? According to a VFF study, a modernization of windows with single glazing is in any case economical. Simply glazed windows were up to approx. Built in 1978 and possess an average U value of only 4.7 W/(m)
²K). An exchange of these windows with a modern, triple glazed window could result in an energy saving of approx. 491 kWh per window and year.

In addition to single-glazed windows, so-called box and composite windows were also installed until 1978. These have an average U value of 2.4 W/(m)
²K). Here too, the exchange can be economically worthwhile.

Between 1978 and 1995 windows with uncoated insulating glass were installed. Although these windows offer significantly better thermal insulation than simply glazed windows, they have only an average U value of 2.7 W/(m)
²K). An exchange of these windows can result in a saving of 222 kWh per window and year. Here, however, as with box and composite windows with a U value of 2.4 W/(m)
²K) to decide from case to case whether the exchange is worthwhile. The new windows will only become economic if the investment for the new windows does not exceed the cost of the saved energy.

If you are unsure whether the exchange of your windows is worthwhile, you can consult an energy consultant.

https://www.deutsche-fensterbau.de/ratgeber-fenster-erneuern/

Note, however, that the possible saving always refers to triple glazed windows. I think the tip with the energy consultant is a good idea because he can judge the actual state and knows what would be useful for you.